High carbon steel sheet superior in fatiugue lifeand manufacturing method thereof

ABSTRACT

The present invention relates to a high carbon steel sheet that is superior in fatigue life and a method of manufacturing the high carbon steel sheet. The high carbon steel sheet includes about 0.75 wt % to about 0.95 wt % of carbon, smaller than about 1.8 wt % of silicon, about 0.1 wt % to about 1.5 wt % of manganese, about 0.1 wt %˜1.0 wt % of chromium, smaller than about 0.02 wt % of phosphorus, smaller than about 0.02 wt % of sulfur, a residual amount of iron, and inevitable impurities. A layer interval between laminar carbides included in the high carbon steel sheet is smaller than about 0.5 μm. The high carbon steel sheet may include a fine pearlite having a lamellar structure. The fine pearlite included in the high carbon steel sheet may have a volume percentage of larger than about 90%. A ratio of length to width of the lamellar structure may be larger than about 10:1.

TECHNICAL FIELD

Embodiments of the present invention relate to a high carbon steel sheetand a method of manufacturing the high carbon steel sheet. Moreparticularly, embodiments of the present invention relate to a highcarbon steel sheet that is superior in fatigue life and a method ofmanufacturing the high carbon steel sheet.

BACKGROUND ART

Recently, a high degree of safety of an automobile has been required.Thus, fatigue life of a spring is also required to manufacture a safeautomobile. The fatigue life of the spring is relatively long when thespring operates in an elastic deformation region. As a result, toincrease the fatigue life of the spring, yield strength of a materialincluded in the spring is increased to increase the elastic deformationregion of the spring. Thereafter, the spring preferably operates withinthe elastic deformation region.

However, the operation region of the spring is sometimes included in aplastic deformation region according to the kind of the spring. In thiscase, a crack may be rapidly generated rather than a case where thespring operates in the elastic deformation region. Here, the fatiguelife is mainly determined by growth of the crack rather than formationof the crack. Thus, in the case of a plate-type band spring having anoperation region included in the plastic deformation region, theplate-type band spring preferably includes a fine structure that iscapable of preventing the growth of the crack in order to increase thefatigue life of the spring.

A conventional steel sheet used to form a spring includes pearlite.However, laminar carbide is relatively large and yield strength thereofis not large. Thus, the fatigue life is not long.

A method of increasing the fatigue life by enlarging the yield strengthof the spring has been suggested. In the method, the yield strength ofthe spring is increased by transforming a fine structure into bainite ata relatively low temperature before performing a cold rolling process toa steel sheet. However, in the case that the plastic deformation regionis included in an operation region of the spring, the fatigue lifedecreases in spite of an increase in the yield strength due to theformed and mixed bainite.

DISCLOSURE

Embodiments of the present invention provide a high carbon steel sheetthat is superior in terms of fatigue life.

Embodiments of the present invention provide a method of manufacturingthe high carbon steel sheet.

In accordance with embodiments of the present invention, a high carbonsteel sheet includes about 0.75 wt % to about 0.95 wt % of carbon,smaller than about 1.8 wt % of silicon, about 0.1 wt % to about 1.5 wt %of manganese, about 0.1 wt %˜1.0 wt % of chromium, smaller than about0.02 wt % of phosphorus, smaller than about 0.02 wt % of sulfur, aresidual amount of iron, and inevitable impurities. A layer intervalbetween laminar carbides included in the high carbon steel sheet issmaller than about 0.5 μm.

The high carbon steel sheet may include a fine pearlite having alamellar structure. The fine pearlite included in the high carbon steelsheet may have a volume percentage of larger than about 90%. A ratio oflength to width of the lamellar structure may be larger than about 10:1

The high carbon steel sheet may further include about 0.05 wt % to about0.25 wt % of vanadium, niobium, molybdenum, titanium, tungsten, orcopper. These may be used alone or in combination. The high carbon steelsheet may further include about 30 ppm to about 120 ppm of nitrogen.

In accordance with embodiments of the present invention, a method ofmanufacturing a high carbon steel sheet is provided. In the method, asteel member including about 0.75 wt % to about 0.95 wt % of carbon,smaller than about 1.8 wt % of silicon, about 0.1 wt % to about 1.5 wt %of manganese, about 0.1 wt % to about 1.0 wt % of chromium, smaller thanabout 0.02 wt % of phosphorus, smaller than about 0.02 wt % of sulfur, aresidual amount of iron, and inevitable impurities is formed. A hotrolling process, a cold rolling process, and an annealing process areperformed to allow the steel member to have a spheroidized cementite andan initial ferrite. A patenting annealing process is performed to theheated steel member after heating the steel member. The patentingannealing process is performed using a solder pot having a maintainedtemperature of about 500° C. to about 530° C. for over about 60 seconds.

The steel member may be heated before the patenting annealing process ata temperature of about 800° C. to about 1100° C. The steel memberfurther may include about 0.05 wt % to about 0.25 wt % of at least oneselected from the group consisting of vanadium, niobium, molybdenum,titanium, tungsten, and copper. The steel member may further includeabout 30 ppm to about 120 ppm of nitrogen.

To manufacture the high carbon steel sheet, a cooling process may beperformed after the patenting annealing process. A cold rolling processmay be the performed such that a reduction ratio of the cold rollingprocess is over about 85%.

In accordance with embodiments of the present invention, a method ofmanufacturing a high carbon steel sheet is provided. In the method, asteel member including about 0.75 wt % to about 0.95 wt % of carbon,smaller than about 1.8 wt % of silicon, about 0.1 wt % to about 1.5 wt %of manganese, about 0.1 wt % to about 1.0 wt % of chromium, smaller thanabout 0.02 wt % of phosphorus, smaller than about 0.02 wt % of sulfur, aresidual amount of iron, and inevitable impurities is formed. A hotrolling process, a cold rolling process and an annealing process areperformed to allow the steel member to have a spheroidized cementite andan initial ferrite. A patenting annealing process is performed to theheated steel member after heating the steel member, the patentingannealing process being performed using a solder pot having a maintainedtemperature of about 500° C. to about 530° C. for over about 20 seconds.

The steel member may be heated before the patenting annealing process ata temperature of about 800° C. to about 1100° C. The steel member mayfurther include about 0.05 wt % to about 0.25 wt % of vanadium, niobium,molybdenum, titanium, tungsten, or copper. These may be used alone or incombination. The steel member may further include about 30 ppm to about120 ppm of nitrogen.

To manufacture the high carbon steel sheet, a cooling process may beperformed after the patenting annealing process. A cold rolling processmay be performed such that a reduction ratio of the cold rolling processis over about 85%.

A high carbon steel sheet according to the embodiments of the presentinvention has a large amount of laminar carbide having a relativelylarge ratio of length to width of over about 10:1. Thus, growth of acrack may be efficiently prevented.

In addition, fatigue life of the high carbon steel sheet may be improvedbecause the growth of the crack is prevented.

Further, in the case that a spring is formed using the high carbon steelsheet according to the embodiments of the present invention, a generatedcrack may not easily grow. Thus, the spring may operate for a relativelylong time even though the spring operates in a plastic deformationregion.

DESCRIPTION OF DRAWINGS

FIG. 1 is a cross-sectional view illustrating a high carbon steel sheetin accordance with an example of the present invention.

FIG. 2 is a cross-sectional view showing a high carbon steel sheet inaccordance with a comparative example.

FIG. 3 shows a shape of a broken high carbon steel sheet according to anexample of the present invention.

FIG. 4 shows a shape of a broken high carbon steel sheet according to acomparative example.

BEST MODE

In accordance with embodiments of the present invention, a high carbonsteel sheet includes about 0.75 wt % to about 0.95 wt % of carbon,smaller than about 1.8 wt % of silicon, about 0.1 wt % to about 1.5 wt %of manganese, about 0.1 wt %˜1.0 wt % of chromium, smaller than about0.02 wt % of phosphorus, smaller than about 0.02 wt % of sulfur, aresidual amount of iron, and inevitable impurities.

In addition, the high carbon steel sheet may further include about 0.05wt % to about 0.25 wt % of vanadium, niobium, molybdenum, titanium,tungsten, or copper. These may be used alone or in combination. The highcarbon steel sheet may further include about 30 ppm to about 120 ppm ofnitrogen.

Here, a layer interval between laminar carbides included in the highcarbon steel sheet is smaller than about 0.5 μm. The high carbon steelsheet may include a fine pearlite having a lamellar structure. The finepearlite included in the high carbon steel sheet may have a volumepercentage of larger than about 90%. A ratio of length to width of thelamellar structure may be larger than about 10:1.

In accordance with embodiments of the present invention, a method ofmanufacturing a high carbon steel sheet is provided. In the method: i) asteel member including about 0.75 wt % to about 0.95 wt % of carbon,smaller than about 1.8 wt % of silicon, about 0.1 wt % to about 1.5 wt %of manganese, about 0.1 wt % to about 1.0 wt % of chromium, smaller thanabout 0.02 wt % of phosphorus, smaller than about 0.02 wt % of sulfur, aresidual amount of iron, and inevitable impurities is formed; ii) a hotrolling process, a cold rolling process and an annealing process isperformed to allow the steel member to have a spheroidized cementite andan initial ferrite; and iii) a patenting annealing process is performedto the heated steel member after heating the steel member at atemperature of about 800° C. to about 1100° C. The patenting annealingprocess is performed using a solder pot having a maintained temperatureof about 500° C. to about 530° C. for over about 60 seconds.Alternatively, the patenting annealing process is performed using asolder pot having a maintained temperature of about 530° C. to about570° C. for about 20 seconds.

The steel member further may include about 0.05 wt % to about 0.25 wt %of vanadium, niobium, molybdenum, titanium, tungsten, or copper. Thesemay be used alone or in combination. The steel member may furtherinclude about 30 ppm to about 120 ppm of nitrogen.

To manufacture the high carbon steel sheet, a cooling process and a coldrolling process may be performed after the patenting annealing processsuch that a reduction ratio of the cold rolling process is over about85%.

Hereinafter, a chemical composition of the high carbon steel sheet isdisclosed.

The content of carbon (C) is about 0.75 wt % to about 0.95 wt %. In thecase that the content of carbon is lower than about 0.75 wt %, hardnessmay increase by a quenching process. Thus, it is difficult to obtainsuperior endurance. In case that the content of the carbon is over about0.95 wt %, residing austenite may be easily formed. In addition, a crackmay be generated by a stress induced transformation when a cold rollingprocess is performed. In addition, toughness and fatigue life of thesteel sheet may be deteriorated.

The content of silicon (Si) is smaller than about 1.8 wt %. In the casethat the content of silicon increases, strength and resistance withrespect to plastic deformation may increase. However, in the case thatthe content of silicon is larger than about 1.8 wt %, the resistancewith respect to the plastic deformation may decrease and decarburizationmay easily occur when a thermal treatment process is performed. Inaddition, quality of a surface may be deteriorated by an increase inscale defects.

The content of manganese (Mn) is about 0.1 wt % to about 1.5 wt %. Inthe case that the content of manganese (Mn) is lower than about 0.1 wt%, red brittleness may occur due to sulfur-iron (FeS) including sulfur(S) and iron (Fe) that are inevitably impurities. On the other hand, inthe case that the content of manganese (Mn) is larger than about 1.5 wt%, toughness may decrease. In addition, in the case that the content ofmanganese (Mn) is larger than about 1.5 wt %, hardenability mayexceedingly increase so that a proceeding velocity of the steel sheet isrequired to be reduced to obtain a fine structure. Thus, yield of thesteel sheet may decrease.

The content of chromium (Cr) may be about 0.1 wt % to about 1.0 wt %.Chromium (Cr) has substantially the same effect as manganese (Mn).Particularly, chromium increases hardenability and strength. Inaddition, chromium may prevent decarburization and graphitization. Inthe case that the content of chromium (Cr) is lower than about 0.1 wt %,it is difficult to obtain sufficient hardenability. In addition, thedecarburization may not be effectively prevented. On the other hand, inthe case that the content of chromium (Cr) is larger than about 1.0 wt%, the hardenability may be exceedingly increased.

The content of sulfur (S) is smaller than about 0.02 wt %. In case thatthe content of sulfur (S) is larger than about 0.02 wt %, toughness maydecrease due to grain boundary segregation.

The content of phosphorus (P) is smaller than about 0.02 wt %. In thecase that the content of phosphorus is larger than about 0.02 wt %,toughness may decrease due to grain boundary segregation.

Molybdenum (Mo), niobium (Nb), titanium (Ti), vanadium (V), or tungsten(W) may be combined with carbon (C) or nitrogen (N) in the steel sheetto generate precipitation hardening. Copper (Cu) may independentlygenerate the precipitation hardening. The content of molybdenum (Mo),niobium (Nb), titanium (Ti), vanadium (V), or tungsten (W) may be about0.05 wt % to about 0.25 wt %. These may be used alone or in combination.The independent or interdependent precipitation hardening due to theabove elements may increase strength of the steel sheet. However, in thecase that the above elements are exceedingly included, a rollingproperty may decrease due to an exceedingly increased hardenabilitybecause an effect of the above elements tends to be saturated. Thus, itis desired to use the above elements selectively. In the case that thecontent of the above element is smaller than about 0.05 wt %, an effectof the precipitation hardening may decrease. On the other hand, in thecase that the content of the above element is larger than about 0.25 wt%, brittleness of the steel sheet may increase when a hot rollingprocess is performed.

The content of nitrogen (N) is about 30 ppm to about 120 ppm. In thecase that the content of nitrogen is smaller than about 30 ppm, aneduced amount of carbon nitride is not sufficient. Thus, improvement ofstrength and resistance with respect to plastic deformation is small. Onthe other hand, in the case that the content of nitrogen (N) is largerthan about 120 ppm, an effect of the precipitation hardening may besaturated and an induced material is exceedingly saturated in a matrix.Thus, toughness of the steel sheet may decrease.

Hereinafter, a fine structure and an effect of preventing a crack fromgrowing in a high carbon steel sheet are described with reference toFIGS. 1 and 2.

FIG. 1 is a cross-sectional view illustrating a high carbon steel sheetin accordance with an example of the present invention. The high carbonsteel sheet has fine pearlite including laminar carbide 101 having arelatively long length in a rolling direction. Here, the laminar carbide101 is formed such that a layer interval between the carbides is lessthan about 0.5 μm. The laminar carbide 101 is formed such that a ratioof length to width is larger than about 10:1. In the case that the layerinterval between the laminar carbides 101 is larger than about 0.5 μm,the number of carbides per volume is small. Thus, fatigue crack growthmay not be efficiently prevented. On the other hand, in the case thatthe ratio of the length to the width is smaller than about 10:1, thecrack may easily grow between the carbides.

Intensity of the carbide is larger than that of a peripheral ferrite.Thus, the crack of the steel sheet may not grow through the carbide. Asa result, the crack tends to grow between the laminar carbides 101.However, the high carbon steel sheet includes a high density of thelaminar carbide having the relatively large ratio of the length to thewidth. Thus, the crack may not grow between the carbides. As a result,the crack formed at a surface of an edge may not easily grow because thecrack has to grow along a complex “A” path “A.” FIG. 2 is across-sectional view showing a high carbon steel sheet in accordancewith Comparative Example 3. The high carbon steel sheet of ComparativeExample 3 includes bainite and fine pearlite. Referring to FIG. 2,laminar carbide 201 included in the high carbon steel sheet ofComparative Example 3 has a relatively small ratio of length to width.Thus, a crack may easily grow between the carbides. That is, the crackmay easily grow along a “B” path so that a fatigue life of the steelsheet may decrease.

That is, the laminar carbide 101 in FIG. 1 included in the high carbonsteel sheet has the relatively large ratio of the length to the height.In addition, the density of the laminar carbide 101 included in the highcarbon steel sheet is relatively large. Thus, the cracks may not beconnected to one another between the carbides. As a result, a growth ofthe fatigue crack may be effectively prevented.

Hereinafter, a method of manufacturing a high carbon steel sheet isdescribed.

A steel member is formed. The steel member includes about 0.75 wt % toabout 0.95 wt % of carbon (C), less than about 1.8 wt % of silicon (Si),about 0.1 wt % to about 1.5 wt % of manganese (Mn), about 0.1 wt % toabout 1.0 wt % of chromium (Cr), less than about 0.02 wt % of phosphorus(P), less than about 0.02 wt % of sulfur (S), a residual amount of iron(Fe), and inevitable impurities. The steel member may further include apredetermined element. The predetermined element may be about 0.05 wt %to about 0.25 wt % of vanadium (V), about 0.05 wt % to about 0.25 wt %of niobium (Nb), about 0.05 wt % to about 0.25 wt % of molybdenum (Mo),about 0.05 wt % to about 0.25 wt % of titanium (Ti), about 0.05 wt % toabout 0.25 wt % of tungsten (W), about 0.05 wt % to about 0.25 wt % ofcopper (Cu), or about 30 ppm to about 120 ppm of nitrogen (N). These maybe used alone or in combination. A chemical composition of the steelsheet is previously described. Thus, any further explanation will beomitted.

A hot rolling process, a cold rolling process, and an annealing processare performed on the steel member so that a steel sheet havingspheroidized cementite and ferrite may be formed. The steel sheet isthen heated at a temperature of about 800° C. to about 1100° C. In thecase that the steel sheet is heated at a temperature of lower than about800° C., the spheroidized cementite may not be fully dissolved in aquenching process. Thus, intensity of a product formed using the steelsheet may decrease after thermal treatment. On the other hand, when thesteel sheet is heated at a temperature of over about 1100° C., surfacedecarburization of a spring steel may occur. In addition, a grain sizeof an austenite phase may increase so that hardening may be exceedinglyrequired. As a result, it is difficult to obtain a fine structure.

Thereafter, a patenting annealing process is performed on the steelsheet at a temperature, i.e., a temperature of a solder pot, maintainedbetween about 500° C. to about 530° C. for about 60 seconds.Alternatively, the patenting annealing process is performed on the steelsheet at a temperature maintained between about 530° C. to about 570° C.for about 20 seconds. In the case that the temperature and the timerequired for the patenting annealing process are not adequate, theaustenite formed in the quenching process may not be transformed intofine pearlite in the solder pot. In this case, the austenite may betransformed into martensite. Alternatively, the austenite may reside.The residing austenite may be changed into a stress-induced martensiteand generate a crack, thereby reducing fatigue life in the cold rollingprocess. In addition, a rolling property may be deteriorated because thestress-induced martensite generates the crack in the cold rollingprocess. On the other hand, in the case that an annealing process isperformed on the steel sheet at a temperature maintained over about 570°C., an interval between layers of the fine pearlite increases. Thus, itis difficult to form a fine structure that is capable of preventinggrowth of a fatigue crack by using a subsequent cold rolling process. Inaddition, intensity may not be easily improved by work hardening.

As described above, the high carbon steel sheet including a finestructure that is superior in fatigue life may be formed by adjustingconditions of the patenting annealing process.

Hereinafter, embodiments of the present invention may be more fullydescribed with reference to examples. It is to be understood that theforegoing examples are illustrative of the present invention and are notto be construed as limited to the specific embodiments disclosed.

Experiment

A carbon steel sheet used for forming a high intensity spring and havingthe above-described composition was prepared. A rolling process was thenperformed on the high carbon steel sheet in which a spheroidizationannealing process was performed so that a plate-shaped coil having athickness of about 1.3 mm to about 1.6 mm was formed. Thereafter, theplate-shaped coil was heated at a quench temperature of about 750° C. toabout 1200° C. for about 2 minutes. A patenting annealing process, i.e.,an austempering process, was then performed using a solder pot having atemperature of about 300° C. to about 650° C. Thereafter, a plate-shapedcoil having a thickness of about 0.23 mm was formed by a rollingprocess. As a result, a material having a uniform thickness in spite ofa different reducing ratio in a cold rolling process was formed. Thecoil having the thickness of about 0.23 mm was slit to have a width ofabout 8 mm required for forming a spring. Thereafter, a bur removingprocess, a shape-forming process, a winding process, and a strain agingprocess were performed to form the spring.

Patenting (or austempering) annealing conditions, quenching annealingconditions, and reduction ratio required for forming a high carbon steelsheet superior in fatigue life for a spring are disclosed in [TABLE 1].

A fatigue test was performed to measure fatigue life of a spring. In thefatigue test, the spring was installed in a fatigue measuring devicewherein rotation and reverse rotation were performed. In order to allowthe spring to operate in a plastic deformation region, the rotation andthe reverse rotation were repeated between a 2^(nd) rotation region to a22^(th) rotation region until the spring was broken. The fatiguemeasuring device repeatedly measured the fatigue life, and the resultsare disclosed in [TABLE 1].

TABLE 1 Patenting Quenching (austempering) annealing annealing Coldprocess process Maintained reduction Fatigue temperature temperaturetime ratio Fine life (° C.) (° C.) (seconds) (%) structure (times)Example 1 900 550 30 85.6 fine pearlite 31,556 Example 2 1000 525 8085.6 fine pearlite 29,032 Comparative 750 550 80 85.6 fine pearlite +17,853 Example 1 pearlite + cementite (not dissolved) Comparative 1200480 80 85.6 upper 7,856 Example 2 bainite + fine pearlite + martensite +surface ferrite(decarburization) Comparative 1050 450 80 85.6 upper14,229 Example 3 bainite + partial fine pearlite Comparative 950 600 8085.6 pearlite 16,886 Example 4 Comparative 950 500 10 85.6 finepearlite + 11,238 Example 5 martensite Comparative 900 550 80 82.4 finepearlite 16,389 Example 6

Referring to Table 1, in the case that the temperature of the quenchingprocess was below about 800° C. (Comparative Example 1), a reversetransformation occurs in a matrix. However, the spheroidized cementitemay not be completely dissolved. The cementite structure that is notdissolved may reside after the austempering annealing process. Thus, aconcentration of carbon measured in the matrix may be insufficient sothat the transformation curve may move forward. As a result, pearlitemay be formed before injection to the solder pot even though thepatenting (or austempering) annealing process is performed at the samecooling speed and some of the austenite may be transformed into finepearlite while the rest of the austenite is maintained in the solderspot. In this case, the amount of the spheroidized cementite that is notdissolved is larger than that of strained laminar carbide. Thus, afatigue crack is not effectively prevented and yield strength may below. As a result, the fatigue life may be relatively short.

In the case that the quenching annealing temperature is over about 1100°C. (Comparative Example 2), the transformation curve may move backbecause of an increase in a grain size of austenite. Thus, a smallamount of martensite may be formed from the austenite remaining in acooling step after passing the solder spot. In addition, ferrite due todecarburization may be found at a surface portion. The martensite maydamage the fatigue life in the plastic transformation region even thoughthe amount of the martensite is small. In addition, in the case thatsurface decarburization occurs, the fatigue strength may be furtherreduced due to a decrease in surface strength.

When the time maintained in the solder spot is short (ComparativeExample 5), remaining austenite may reside or the fatigue life mayreduced due to the martensite formed in a cooling step after passing thesolder spot.

In the case that the maintained temperature of the patenting (oraustempering) annealing temperature is changed into a low temperature(Comparative Example 3), the yield strength measured after the annealingmay increase.

The fatigue life is relatively long in an elastic deformation region.However, in the case that the plastic deformation region is included inan operation region, the fatigue life may be short. This is becausebainite carbide is not effective to prevent a growth of the crack.

In the case that the plastic deformation region is included in thespring operation region (i.e., a stress applied to a surface acts largerthan the yield strength), a crack may be easily generated. Thus, thefatigue life may be largely determined by the growth of the crack.Referring to FIG. 2, in the case that carbide having a relatively smallratio of length to width is mixed in a fine structure, the growth of acrack may not be prevented. Thus, a length of a path where the crackgrows may become short so that the fatigue life may become low in thespring operation region including the plastic deformation region.

FIGS. 3 and 4 show shapes of springs according to Example 1 andComparative Example 3 broken after the fatigue test.

FIG. 3 shows a fatigue crack that is generated after the fatigue testperformed to the high carbon steel sheet according to an example of thepresent invention. FIG. 4 shows a fatigue crack generated after thefatigue test performed to a high carbon steel sheet according to acomparative example. Referring to FIGS. 3 and 4, a step-shaped break isgenerated according to the example of the present invention. However, aline-shaped break is generated according to the comparative example.That is, generation and a growth of a crack may be efficiently preventedin the example of the present invention rather than the comparativeexample.

In the case that the maintained patenting (or austempering) temperatureis above 570° C. (Comparative Example 4), a pearlite fine structure maybe generated. Carbon may be largely strained due to a relatively highreduction ratio. However, the amount of laminar carbide per volume isrelatively small as opposed to in the example of the present invention.Thus, the growth of the fatigue crack may not be efficiently preventedas opposed to in the example of the present invention.

In the case that a cold reduction ratio is not sufficient (i.e.,Comparative Example 6) in spite of the quenching annealing temperature,and the temperature of the patenting (or austempering) and time areincluded in proper ranges, the fatigue life is smaller those of theexamples of the present invention. However, the fatigue life is largerthan those of other comparative examples.

As shown in Table 1, the fatigue life according to the examples of thepresent invention is superior to the comparative examples. This isbecause the high carbon steel sheet according to the example of thepresent invention has the large amount of strained laminar carbidehaving a relatively large ratio of length to width of the finestructure. The laminar carbide may prevent growth of the fatigue crack.In the case that the plastic deformation region is included in thespring operation region (i.e., a stress applied to a surface acts largerthan the yield strength), a crack may be easily generated. Thus, thefatigue life may be mainly determined by the growth of the crack. As aresult, the fatigue life may be varied by the fine structure that iscapable of preventing the growth of the crack and the length of a pathwhere the fatigue crack grows.

The high carbon steel sheet according to the examples of the presentinvention includes a fine structure in which a large amount of finepearlite that is strained by a rolling process per volume is arranged.The fine structure is advantageous to increase the fatigue life.

INDUSTRIAL APPLICABILITY

A high carbon steel sheet according to the embodiments of the presentinvention has the large amount of laminar carbide having a relativelylarge ratio of length to width of over about 10:1. Thus, growth of acrack may be efficiently prevented.

In addition, fatigue life of the high carbon steel sheet may be improvedbecause the growth of the crack is prevented.

Further, in the case that a spring is formed using the high carbon steelsheet according to the embodiments of the present invention, a generatedcrack may not easily grow. Thus, the spring may operate for a relativelylong time even though the spring operates in a plastic deformationregion.

1. A high carbon steel sheet comprising about 0.75 wt % to about 0.95 wt% of carbon, smaller than about 1.8 wt % of silicon, about 0.1 wt % toabout 1.5 wt % of manganese, about 0.1 wt %˜1.0 wt % of chromium,smaller than about 0.02 wt % of phosphorus, smaller than about 0.02 wt %of sulfur, a residual amount of iron, and inevitable impurities, whereina layer interval between laminar carbides included in the high carbonsteel sheet is smaller than about 0.5 μm.
 2. The high carbon steel sheetof claim 1, wherein the high carbon steel sheet includes a fine pearlitehaving a lamellar structure.
 3. The high carbon steel sheet of claim 2,wherein the fine pearlite included in the high carbon steel sheet has avolume percentage of larger than about 90%.
 4. The high carbon steelsheet of claim 2, wherein a ratio of length to width of the lamellarstructure is larger than about 10:1.
 5. The high carbon steel sheet ofclaim 1, further comprising about 0.05 wt % to about 0.25 wt % of atleast one selected from the group consisting of vanadium, niobium,molybdenum, titanium, tungsten, and copper.
 6. The high carbon steelsheet of claim 5, further comprising about 30 ppm to about 120 ppm ofnitrogen.
 7. A method of manufacturing a high carbon steel sheet, themethod comprising: forming a steel member including about 0.75 wt % toabout 0.95 wt % of carbon, smaller than about 1.8 wt % of silicon, about0.1 wt % to about 1.5 wt % of manganese, about 0.1 wt % to about 1.0 wt% of chromium, smaller than about 0.02 wt % of phosphorus, smaller thanabout 0.02 wt % of sulfur, a residual amount of iron, and inevitableimpurities; performing a hot rolling process, a cold rolling process andan annealing process to allow the steel member to have a spheroidizedcementite and an initial ferrite; and performing a patenting annealingprocess to the heated steel member after heating the steel member, thepatenting annealing process being performed using a solder pot having amaintained temperature of about 500° C. to about 530° C. for over about60 seconds.
 8. The method of claim 7, wherein the steel member is heatedbefore the patenting annealing process at a temperature of about 800° C.to about 1100° C.
 9. The method of claim 7, wherein the steel memberfurther includes about 0.05 wt % to about 0.25 wt % of at least oneselected from the group consisting of vanadium, niobium, molybdenum,titanium, tungsten, and copper.
 10. The method of claim 9, wherein thesteel member further includes about 30 ppm to about 120 ppm of nitrogen.11. The method of claim 7, further comprising: performing a coolingprocess after the patenting annealing process; and performing a coldrolling process such that a reduction ratio of the cold rolling processis over about 85%.
 12. A method of manufacturing a high carbon steelsheet, the method comprising: forming a steel member including about0.75 wt % to about 0.95 wt % of carbon, smaller than about 1.8 wt % ofsilicon, about 0.1 wt % to about 1.5 wt % of manganese, about 0.1 wt %to about 1.0 wt % of chromium, smaller than about 0.02 wt % ofphosphorus, smaller than about 0.02 wt % of sulfur, a residual amount ofiron, and inevitable impurities; performing a hot rolling process, acold rolling process, and an annealing process to allow the steel memberto have spheroidized cementite and initial ferrite; and performing apatenting annealing process to the heated steel member after heating thesteel member, the patenting annealing process being performed using asolder pot having a maintained temperature of about 500° C. to about530° C. for over about 20 seconds.
 13. The method of claim 12, whereinthe steel member is heated before the patenting annealing process at atemperature of about 800° C. to about 1100° C.
 14. The method of claim12, wherein the steel member further includes about 0.05 wt % to about0.25 wt % of at least one selected from the group consisting ofvanadium, niobium, molybdenum, titanium, tungsten, and copper.
 15. Themethod of claim 14, wherein the steel member further includes about 30ppm to about 120 ppm of nitrogen.
 16. The method of claim 12, furthercomprising: performing a cooling process after the patenting annealingprocess; and performing a cold rolling process such that a reductionratio of the cold rolling process is over about 85%.